In recent years, as an insulating protective film of a flexible circuit board, polyimide resins, polyamideimide resins, polyurethane resins, epoxy resins and the like are used. When preparing a circuit board, a photosensitive material is used to form a patterned circuit, a protective layer of the circuit board surface and the patterned circuit, an interlayer insulation film and the like. Photosensitive materials can be broadly classified into liquid type and film type. In either case, the cured film is required to have excellent flexibility, bending resistance, low warping and the like in addition to excellent heat resistance and electric properties. Polyimides are widely used because of their excellent heat resistance and electric properties; however, since polyimides themselves are rigid, a variety of methods have been proposed to improve the rigidity. One type of the methods is so-called mix-type method in which a polyimide or its precursor, polyamic acid, is mixed with a resin component having flexibility, and the other type is so-called modification-type method in which a flexible component such as a polysiloxane structure is introduced into the polyimide main chain.
As a mixed-type photosensitive material, Patent Document 1 discloses a dry film resist comprising a polyimide resin and a (meth)acrylic resin. However, since the (meth)acrylic compound is often added in a large amount such that curing property and low-temperature laminating property can be imparted, depending on the (meth)acrylic compound, the heat resistance of the dry film resist may be lowered and excellent characteristics of the polyimide may be impaired. Further, the compatibility between the resins also needs to be considered. When the compatibility is extremely bad, the photosensitivity becomes largely variable on the surface, which may result in an impaired reliability in the pattern accuracy, film thickness and the like.
As a modified-type photosensitive material, Patent Document 2 discloses a composition comprising a siloxane-modified polyimide derived from diaminopolysiloxane. This siloxane-modified polyimide has excellent heat resistance and electric insulation property and is also excellent in low warping because of its low elasticity. Yet, there are also problems in that the surface of the cured film does not have sufficient adhesion due to the polysiloxane component and that cyclic siloxane is generated and released during high-temperature heating.
Meanwhile, although they are not photosensitive materials, Patent Document 3 discloses a polycarbonate-modified polyimide resin and Patent Document 4 discloses a composition comprising an ester-modified polyimide resin. These compositions are excellent in low warping and in these compositions, those problems seen in siloxane-modified polyimides, such as insufficient adhesion of the cured film surface and generation of cyclic siloxane, were improved. It is speculated that photosensitivity can be imparted to these modified resins by adding a photosensitive resin composition such as a (meth)acrylic resin as described in Patent Document 1; however, in such a composition having a non-photosensitive resin as its major constituting material, problems that are similar to those of a mixed-type material, such as inconsistent photosensitivity on the surface, tend to arise. In addition, with each resin alone, the chemical resistance tends to be low; therefore, as described in these Patent Documents, an epoxy must be added; however, this lowers the storage stability.
Accordingly, a method in which a modified resin is imparted with photosensitivity, not a mixing-type system, is speculated to be the most appropriate. Patent Document 5 discloses a composition which comprises a polyimide resin having an ether imide skeleton, polycarbonate skeleton and urethane bond, as well as a developing group in the side chain and a photosensitive group at the main chain terminal. In this composition, a (meth)acryloyl group is used as the photosensitive group, and the composition is composed of a (meth)acrylate compound, a photopolymerization initiator and a thermal polymerization initiator. However, since this composition is a so-called negative-type photosensitive composition, although the cured film obtained therefrom has an improved solvent resistance, an epoxy or isocyanate compound must be added as a curing agent, so that there is a problem of bad storage stability. In addition, since it is required to introduce a (meth)acryloyl group to a terminal or the like of the polyimide chain and gelation easily occurs during the polyimide synthesis, the synthesis of the composition is difficult. That is, attention must be paid not to allow the heat applied during the synthesis to trigger a reaction of the (meth)acryloyl group, and the management of the viscosity is also not easy. Besides, since an addition of a polymerization inhibitor inhibits the curing reaction in the exposure process and the resulting product is thus affected, the above-described problems cannot be easily resolved.
Considering the storage stability as well, it is speculated rather easier to employ a positive-type photosensitive resin represented by novolac resins as the resin composition. Specifically, a method in which a hydroxyl group or carboxyl group is introduced into the main chain of a modified polyimide is suitable. However, as described in Patent Documents 3 and 4, since a modified polyimide is synthesized by a reaction between a tetracarboxylic dianhydride and a diisocyanate, hydroxyl group and carboxyl group cannot be introduced. That is, since hydroxyl group and carboxyl group react with isocyanate, they are consumed by cross-linking reaction and the like. Similarly, since the composition of Patent Document 5 is also similarly obtained by allowing a hydroxyl group-terminated ether imide oligomer with a diol compound, (a compound having a (meth)acryloyl group) and a diisocyanate compound, it is difficult to protect the intended hydroxyl group.